The true unity of all who call on Christ (and the whole human family for that matter.)
And what is true unity?
Pope John Paul II explains:
"This unity, which the Lord has bestowed on his Church and in which he wishes to embrace all people, is not something added on, but stands at the very heart of Christ’s mission. Nor is it some secondary attribute of the community of his disciples. Rather, it belongs to the very essence of this community. God wills the Church, because he wills unity, and unity is an expression of the whole depth of his* agape*.
In effect, this unity bestowed by the Holy Spirit does not merely consist in the gathering of people as a collection of individuals. It is a unity constituted by the bonds of the profession of faith, the sacraments and hierarchical communion.10 The faithful are one because, in the Spirit, they are in* communion* with the Son and, in him, share in his communion with the Father: “Our fellowship is with the Father and with his Son Jesus Christ" (1 Jn 1:3). For the Catholic Church, then, thecommunion of Christians is none other than the manifestation in them of the grace by which God makes them sharers in his own* communion*, which is his eternal life. Christ’s words “that they may be one” are thus his prayer to the Father that the Father’s plan may be fully accomplished, in such a way that everyone may clearly see “what is the plan of the mystery hidden for ages in God who created all things” (Eph 3:9). To believe in Christ means to desire unity; to desire unity means to desire the Church; to desire the Church means to desire the communion of grace which corresponds to the Father’s plan from all eternity. Such is the meaning of Christ’s prayer: "Ut unum sint”.
“The unity willed by God can be attained only by the adherence of all to the content of revealed faith in its entirety. In matters of faith, compromise is in contradiction with God who is Truth. In the Body of Christ, “the way, and the truth, and the life” (
Jn 14:6), who could consider legitimate a reconciliation brought about at the expense of the truth? The Council’s Declaration on Religious Freedom
Dignitatis Humanae attributes to human dignity the quest for truth, “especially in what concerns God and his Church”,
33 and adherence to truth’s demands. A “being together” which betrayed the truth would thus be opposed both to the nature of God who offers his communion and to the need for truth found in the depths of every human heart.”
While the Church has employed different means towards this end at different times, the end also remains the same.
For additional Magisterial teaching on this subject, I recommend these texts:
Leo XIII,
Praeclara Gratulationis Publicae (The Reunion of Christendom) June 20, 1894
Pius XI,
Mortalium Animos (On Religious Unity) January 6, 1928
Pius XII,
Summi Pontificatus (On the Unity of Human Society) October 10, 1939
Bl. John XXIII,
Ad Petri Cathedram (On Truth, Unity and Peace in a Spirit of Charity) June 29, 1959
Second Vatican Council,
Unitatis Redintegratio, (Decree on Ecumenism), November 21, 1964
John Paul II,
Ut Unum Sint (On Commitment to Ecumenism) May 25, 1995